A man who raped his daughter was able to be employed as a social worker after concealing his conviction.

Another man who called disabled people "dumb" and "homo" was also engaged in the sector, while a third worker financially exploited a client and was ordered to pay $100,000 in reparation.

All of the so-called "social workers" were unregistered and therefore not vetted by the sector's professional standards body or able to be investigated and disciplined through its complaints tribunal.

They were just three in a long list of unsuitable candidates employed to work with vulnerable clients because there is no law requiring social workers in New Zealand to belong to the Social Worker's Registration Board.

Complaints against unregistered workers have prompted the board and several politicians to call for mandatory registration but the Government says it won't happen.

At least 10 serious complaints have been received by the board this year, but chair Toni Hocquard said it had no option but to note them and carry on because the workers were not within their jurisdiction.

"The concerning issue is that there is evidence to show that allowing unqualified, incompetent, inexperienced individuals to work with the most vulnerable members of society causes significant harm," Hocquard said.

"The evidence is seen daily in the media, the courts, the hospitals and morgues throughout New Zealand."

Hocquard said the number of complaints received were just the "tip of the iceberg", as most people needing social workers were already vulnerable and unlikely to complain.

There are about 14,000 social workers in New Zealand. Only 4000 are registered, with the board assessing that around another 4000 more would fit the criteria, while 6000 have limited training or experience.

Those unregistered include government employees, such as 30 per cent of Child, Youth and Family's (CYF) frontline staff.

Unregistered workers are not faced with the same scrutiny, such as police vetting and identity checks, and therefore can "slip under the radar", the board says.

For example, the "social worker" who raped his daughter was able to find work as a volunteer in Hawke's Bay after leaving prison due to a legal loophole. Because he had name suppression - automatic because the victim was his daughter and therefore identifiable - his conviction was not on his record. If he had been required to be registered, a full police check would have revealed the conviction, the board said.

Likewise the other cases - of exploitation and abuse - would mean the workers were struck off and therefore unable to find employment as social workers again.

The issue of compulsory registration came before the Government last year as part of the White Paper on vulnerable children, but a decision was made against mandatory registration. Labour MP Rajen Prasad said that decision was disappointing and it was now time for the government to "bite the bullet".

He said it was not credible for vulnerable people to be put under the care of social workers who were sometimes "neither trained nor accountable".

Green MP Holly Walker said the party was particularly concerned about the number of unregistered CYF employees.

"They're the largest group of social workers in the country so it's really important that they are accountable to some sort of external body," she said.

Associate Minister for Social Development Chester Borrows said the Children's Action Plan, formed out of the White Paper, encouraged social workers to register with the Social Workers Registration Board.

"Where possible I encourage other professional groups of social workers to set a similar goal," he said.